applied sciences

Article

Evaluation of Anti-Wear Properties of MetalworkingFluids Enhanced with Halloysite Nanotubes

Laura Peña-Parás 1,* ID , José Antonio Sánchez-Fernández 2, Carlos Rafael Martínez 1,José Abraham Ontiveros 1, Karla Itzel Saldívar 1, Luis Manuel Urbina 1, Moisés Jair Arias 1,Patricio García-Pineda 1 and Brenda Castaños 1

1 Departamento de Ingeniería, Universidad de Monterrey,66238 San Pedro Garza García, Nuevo León, Mexico; carlos.martinezc@udem.edu (C.R.M.);jose.ontiverosg@udem.edu (J.A.O.); karla.saldivar@udem.edu (K.I.S.); luis.urbina@udem.edu (L.M.U.);moises.arias@udem.edu (M.J.A.); patricio.garcia@udem.edu (P.G.-P.); brenda.castanos@udem.edu (B.C.)

2 Escuela de Ingeniería y Ciencias, Tecnológico de Monterrey, Campus Monterrey, Eugenio Garza Sada 2501,Col. Tecnológico, 64849 Monterrey, Nuevo León, Mexico; asanfer@itesm.mx

* Correspondence: laura.pena@udem.edu; Tel.: +52-8182151000 (ext. 1894)

Received: 22 August 2017; Accepted: 25 September 2017; Published: 3 October 2017

Featured Application: nanoparticle additives for improving tribological properties of metalworkingcutting fluids, enhancing tool life and surface finish in machining processes.

Abstract: The study of nanoparticles as additives for metalworking fluids (MWFs) with applicationsin the metal removal processes, or machining, has received increasing attention due to the possibleenhancements on tribological properties. In this study, low-cost and environmentally friendlynanoparticle additives of halloysite clay nanotubes (HNTs) were dispersed in metalworking fluidsutilized for milling processes. Concentrations of 0.01, 0.05, 0.10 wt. % were incorporated intoa mineral oil (MO) and a semi-synthetic fluid (SF) by ultrasonication. The anti-wear propertiesof metalworking nanofluids were characterized with a T-05 block-on-ring tribotester at a contactpressure of 0.5 GPa. Surface roughness of worn block materials was obtained with an optical 3Dsurface measurement system. Results showed that at a concentration of 0.10 wt. % HNTs block massloss was lowered by 24% for the MO + HNTs nanofluids. For the SF + HNTs, a reduction of 63%and 32% in wear mass loss and coefficient of friction (COF), respectively, were found at the sameconcentration. The tribological enhancing mechanism for the applied contact pressure was proposedto be due to a reduction of the area of contact and nanoparticle sliding between surfaces with no HNTdeposition, evidenced by energy dispersive spectrometry (EDS). Furthermore, surface roughnessstudies of worn blocks showed smoother surfaces with lower groove density with the addition ofnanoparticle additives. The results of this study demonstrate that HNTs can improve the lubricity ofmetalworking cutting fluids used for machining processes, enhancing tool life and providing bettersurface finish of products.

Keywords: nanofluids; metalworking fluids; halloysite nanotubes; wear; coefficient of friction;surface roughness

1. Introduction

Metalworking fluids (MWFs) are used in metal removal processes such as cutting, milling,and grinding to reduce friction between the tool and the workpiece, increase tool life by reducingwear, and improve surface finish [1–4]. MWFs with nanoparticle additives have been widelyinvestigated for the purpose of reducing the mechanical friction and wear between componentsobtaining very promising results [5–14]. For example, the wear scar diameters (WSDs) of a synthetic

Appl. Sci. 2017, 7, 1019; doi:10.3390/app7101019 www.mdpi.com/journal/applsci

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fluid and base oil for machining applications were lowered by 10% and 86%, respectively, with0.01 wt. % CuO nanoparticles [10]. Similarly, Mosleh et al. [15] dispersed MoS2, WS2, and hexagonalboron nitride (hBN) nanoparticles in MWFs obtaining wear reductions of up to 66%. This wasattributed to nanoparticles filling surface valleys and the shearing of trapped nanoparticles at theinterface of contacting surfaces. Another study by Mosleh et al. [12] found that nanoparticlesalso deagglomerate metallic transfer films further reducing friction and wear. Zhang et al. [16]dispersed CaCO3 nanoparticles in a poly-alpha-olefin (PAO) base oil improving load-carrying capacity,and reducing friction and wear. Significant friction reductions in boundary and mixed lubricationregimes were obtained by Kalin et al. [17] through the formation of a protective film by dispersingMoS2 nanotubes into PAO. Moreover, Wu et al. [5] achieved reductions in friction and wear by CuO,TiO2, and nano-diamond nanoparticles due to a rolling effect provided by their spherical shape.

The addition of nanoparticles to MWFs has also demonstrated improvements in tool life andsurface finish of products. Exfoliated graphene nanoparticles added to a vegetable oil reduced edgechipping of cutting tool edges in ball milling experiments [18]. A cutting fluid with dispersed CuOnanoparticles developed by Vázquez et al. [11] was employed in a Computer Numerical Control (CNC)gear generator obtaining an improvement on the surface finish of workpieces of ~52%. Nanoparticlesof SiO2 blended into a mineral oil investigated for plain bearings by Sia et al. [19,20] reduced frictiontemperatures by acting as rolling and sliding particles between surfaces. Furthermore, a nanolubricantcontaining MoS2 nanoparticles for end-milling of AL6061-T6 developed by Rahmati et al. [21] improvedthe quality of the machined surfaces by the rolling, mending, and polishing tribological mechanismsprovided by MoS2 in the tool-workpiece interface.

Halloysite nanotubes (HNTs), with a molecular formula of Al2Si2O5(OH)4·nH2O, are hollow claymultiwalled tubular nanoparticles with lengths of 50–5000 nm, diameters of 20–200 nm, and innerlumen of 10–70 nm that naturally form in the Earth [22–24]; however, their dimensions may varydepending on the deposit [25]. These nanoparticles have negatively charged SiO2 groups on theoutside of the layers and positively charged Al(OH)3 on the inner lumen [26–28]. HNTs are non-toxiceven at high concentrations, and are much less expensive compared to other tubular-like nanoparticlessuch as carbon nanotubes (CNTs) [22]. Due to these characteristics they have been studied for diverseapplications, such as for preparing colloids with biopolymers providing tunable properties [29],for drug transport and delivery [28], as nanocontainers of additives for paper preservation [30],for obtaining inorganic reverse micelles for the entrapment of antibacterial agents [31], and forcapturing hydrocarbon and aromatic oils [32].

Recently, our group demonstrated that HNTs dispersed in metal-forming lubricants tested underextreme pressure conditions are able to increase the load-carrying capacity, scuffing load, and seizureload (load for lubricant film breakdown) under contacts pressures of 3.5–3.9 GPa [33]. At this highcontact pressure the tribological enhancing mechanism of HNTs was due to a possible exfoliation oftheir outer sheets and deposition on the surfaces.

In this work, two MWFs commonly used in the metal-mechanic industry, a semi-synthetic fluid(SF) and a mineral oil (MO), were reinforced with varying concentrations of HNTs and tested under acontact pressure of 0.5 GPa (lower than our previous work). The tribological properties of coefficient offriction (COF) and wear mass loss of nanofluids were determined with a block-on-ring test performedat room temperature. Surface roughness of tested materials was characterized after the test to study theinfluence of these nanoparticles on the surface finish of blocks. The proposed tribological enhancingmechanism of this study was determined by energy dispersive spectrometry (EDS).

2. Materials and Methods

2.1. Materials

Nanofluids were prepared by adding HNTs (purchased from Sigma-Aldrich, St. Louis, MO,USA) in concentrations of 0.01, 0.05, and 0.10 wt. % within a MO and a SF. These concentrations

Appl. Sci. 2017, 7, 1019 3 of 15

were selected based on a previous study by our group [33]. Figure 1a shows a micrograph with themorphology on HNTs obtained by scanning electron microscopy (SEM). Dimension measurements ofHNTs obtained by SEM are observed in Figure 1b. Elemental analysis of HNTs in Figure 1c shows Al,Si, and O in concentrations of 26%, 26%, and 43%, respectively. Also, Table 1 presents the propertiesof nanoparticles and MWFs. Nanoparticles were dispersed in both MWFs using an Ultra Turraz T10homogenizer for 5 min at room temperature (25 ◦C), followed by 5 min of ultrasonication with aCole-Parmer 750-watt ultrasonic homogenizer.

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morphology on HNTs obtained by scanning electron microscopy (SEM). Dimension measurements

of HNTs obtained by SEM are observed in Figure 1b. Elemental analysis of HNTs in Figure 1c shows

Al, Si, and O in concentrations of 26%, 26%, and 43%, respectively. Also, Table 1 presents the

properties of nanoparticles and MWFs. Nanoparticles were dispersed in both MWFs using an Ultra

Turraz T10 homogenizer for 5 min at room temperature (25 °C), followed by 5 min of ultrasonication

with a Cole-Parmer 750-watt ultrasonic homogenizer.

Figure 1. (a) Scanning electron micrograph of halloysite nanotubes (HNTs); (b) Micrograph of HNTs

at higher magnification indicating nanoparticle dimensions; (c) elemental analysis of HNTs showing

high concentrations of Al, Si, and O.

Table 1. Material properties.

Materials Properties

Metalworking fluids Density (15 °C) Viscosity (40 °C)

Semi-synthetic fluid (SF) 0.970 g/cm3 75.0 mm2/s

Mineral oil (MO) 0.871 g/cm3 33.2 mm2/s

Nanoparticle

Halloysite clay nanotubes (HNTs) Morphology: tubular; diameter: 28.5–135 nm × length: 1–3 μm

Test blocks

AISI 1018 Dimensions: 6.35 × 10 × 15.75 mm, 71 HRB, Ra = 0.45 μm

Chemical composition: 0.14–0.20% C, 0.60–0.90% Mn, <0.04% P, 0.05% S, Fe Balance

Test rings

AISI D2 Diameter: 35 mm, 58 HRC, Ra = 0.65 μm

Chemical composition: 1.5511.5% Cr, 0.90% Mo, 0.80% V, Fe Balance

Figure 1. (a) Scanning electron micrograph of halloysite nanotubes (HNTs); (b) Micrograph of HNTs athigher magnification indicating nanoparticle dimensions; (c) elemental analysis of HNTs showing highconcentrations of Al, Si, and O.

Table 1. Material properties.

Materials Properties

Metalworking fluids Density (15 ◦C) Viscosity (40 ◦C)

Semi-synthetic fluid (SF) 0.970 g/cm3 75.0 mm2/s

Mineral oil (MO) 0.871 g/cm3 33.2 mm2/s

Nanoparticle

Halloysite clay nanotubes (HNTs) Morphology: tubular; diameter: 28.5–135 nm × length: 1–3 µm

Test blocks

AISI 1018 Dimensions: 6.35 × 10 × 15.75 mm, 71 HRB, Ra = 0.45 µmChemical composition: 0.14–0.20% C, 0.60–0.90% Mn, <0.04% P, 0.05% S, Fe Balance

Test rings

AISI D2 Diameter: 35 mm, 58 HRC, Ra = 0.65 µmChemical composition: 1.5511.5% Cr, 0.90% Mo, 0.80% V, Fe Balance

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Figure 2 shows the hydrodynamic diameter size distribution of HNTs over time for MO and SFnanofluids, where measurements were taken after following the dispersion method. These tests wereperformed with a Malvern Zetasizer Nano ZS using dynamic light scattering after 3, 6, and 9 min.In Figure 2a the HNTs hydrodynamic diameter size distribution increases rapidly with time for MOnanofluids, suggesting lower stability in this fluid, likely due to its lower viscosity (33.2 mm2/s)compared to SF (75.0 mm2/s). For SF nanofluids the size distribution remains stable with very similarcurves (Figure 2b). Figure 2c shows a plot of the average HNTs hydrodynamic diameter size over time,indicating the lower stability in MO.

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Figure 2 shows the hydrodynamic diameter size distribution of HNTs over time for MO and SF

nanofluids, where measurements were taken after following the dispersion method. These tests

were performed with a Malvern Zetasizer Nano ZS using dynamic light scattering after 3, 6, and 9

min. In Figure 2a the HNTs hydrodynamic diameter size distribution increases rapidly with time for

MO nanofluids, suggesting lower stability in this fluid, likely due to its lower viscosity (33.2 mm2/s)

compared to SF (75.0 mm2/s). For SF nanofluids the size distribution remains stable with very similar

curves (Figure 2b). Figure 2c shows a plot of the average HNTs hydrodynamic diameter size over

time, indicating the lower stability in MO.

Figure 2. (a) Hydrodynamic size distribution of HNTs over time for MO + 0.01 wt. % HNT; (b)

Hydrodynamic size distribution of HNTs over time for SF + 0.01 wt. % HNTs; (c) Average HNTs

hydrodynamic diameter over time comparison for MO + 0.01 wt. % HNTs and SF + 0.01 wt. % HNTs.

Lower stability is found for MO nanofluids.

2.2. Tribological Tests

Tribological characterization of nanofluids was performed with a T-05 block-on-ring tribotester

shown in Figure 3a, according to ASTM G77. The schematic of the block-on-ring configuration used

for this study indicating the direction of the applied load is shown in Figure 3b. Figure 3c shows the

dimensions of test block and ring samples.

In this test a normal load is applied to the system (Figure 3b), where the test block is locked into

position and the ring rotates with a selected speed and load. Approximately 50 mL of nanofluids are

placed in the lubricant container so the materials are lubricated during testing. The block and ring

materials were an AISI 1018 steel and an AISI D2 steel, respectively (Table 1). Testing was done at

room temperature (25 °C) under the following parameters: a normal load of 750 N (contact pressure

of ~0.5 GPa), 200 rpm, and a duration of the run of 3600 s. Five tests were performed for each

nanofluid; after each test wear mass loss of blocks was recorded and COF was obtained.

Figure 2. (a) Hydrodynamic size distribution of HNTs over time for MO + 0.01 wt. % HNT;(b) Hydrodynamic size distribution of HNTs over time for SF + 0.01 wt. % HNTs; (c) Average HNTshydrodynamic diameter over time comparison for MO + 0.01 wt. % HNTs and SF + 0.01 wt. % HNTs.Lower stability is found for MO nanofluids.

2.2. Tribological Tests

Tribological characterization of nanofluids was performed with a T-05 block-on-ring tribotestershown in Figure 3a, according to ASTM G77. The schematic of the block-on-ring configuration usedfor this study indicating the direction of the applied load is shown in Figure 3b. Figure 3c shows thedimensions of test block and ring samples.

In this test a normal load is applied to the system (Figure 3b), where the test block is locked intoposition and the ring rotates with a selected speed and load. Approximately 50 mL of nanofluids areplaced in the lubricant container so the materials are lubricated during testing. The block and ringmaterials were an AISI 1018 steel and an AISI D2 steel, respectively (Table 1). Testing was done atroom temperature (25 ◦C) under the following parameters: a normal load of 750 N (contact pressure of~0.5 GPa), 200 rpm, and a duration of the run of 3600 s. Five tests were performed for each nanofluid;after each test wear mass loss of blocks was recorded and COF was obtained.

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Appl. Sci. 2017, 7, 1019 5 of 15

Figure 3. (a) T-05 Block-on-ring tribotester; (b) Schematic of the block-on-ring setup showing the

direction of the applied force; (c) Dimensions of the test block and ring.

2.3. Surface Characterization

Surface roughness characteristics of average roughness (Ra), root-mean-square roughness (Rq),

and mean roughness depth (Rz) were characterized with an Alicona IF-EdgeMaster optical 3D

surface measurement system (Alicona Imaging GmbH, Raaba/Graz, Austria). A Tescan Vega3 SB

Scanning Electron Microscope (SEM) equipped with EDS (Tescan, Brno—Kohoutovice, Czech

Republic) was used to identify the tribological mechanism provided by HNTs.

3. Results and Discussion

3.1. Tribological Results

Block mass loss presented in Figure 4 was quantified by weighting each block before and after

every test. In general, the SF + HNT nanofluids exhibited the best tribological behavior with much

lower wear mass loss compared to MO + HNT nanofluids. This was likely due to the higher viscosity

of SF compared to MO (75.0 mm2/s vs. 33.2 mm2/s) that was able to provide a better lubricant film

and nanoparticle stability, as shown in Figure 2. For MO + HNT nanofluids the lowest wear mass

loss was 8.05 mg for MO + 0.10 wt. % HNTs, giving an enhancement of 24% compared to the unfilled

MO. For SF + HNT nanofluids improvements of 6%, 36%, and 63% were found for 0.01, 0.05, and

0.1 wt. % HNTs, respectively. Only 2.5 mg of mass loss was obtained for SF + 0.1 wt. % HNTs

compared to 6.9 mg for the unfilled SF. It is also observed that the standard deviation of the wear

mass results diminishes with increasing nanoparticle concentration, thus increasing nanolubricant

reliability.

Figure 3. (a) T-05 Block-on-ring tribotester; (b) Schematic of the block-on-ring setup showing thedirection of the applied force; (c) Dimensions of the test block and ring.

2.3. Surface Characterization

Surface roughness characteristics of average roughness (Ra), root-mean-square roughness (Rq),and mean roughness depth (Rz) were characterized with an Alicona IF-EdgeMaster optical 3D surfacemeasurement system (Alicona Imaging GmbH, Raaba/Graz, Austria). A Tescan Vega3 SB ScanningElectron Microscope (SEM) equipped with EDS (Tescan, Brno—Kohoutovice, Czech Republic) wasused to identify the tribological mechanism provided by HNTs.

3. Results and Discussion

3.1. Tribological Results

Block mass loss presented in Figure 4 was quantified by weighting each block before and afterevery test. In general, the SF + HNT nanofluids exhibited the best tribological behavior with muchlower wear mass loss compared to MO + HNT nanofluids. This was likely due to the higher viscosityof SF compared to MO (75.0 mm2/s vs. 33.2 mm2/s) that was able to provide a better lubricantfilm and nanoparticle stability, as shown in Figure 2. For MO + HNT nanofluids the lowest wearmass loss was 8.05 mg for MO + 0.10 wt. % HNTs, giving an enhancement of 24% compared to theunfilled MO. For SF + HNT nanofluids improvements of 6%, 36%, and 63% were found for 0.01, 0.05,and 0.1 wt. % HNTs, respectively. Only 2.5 mg of mass loss was obtained for SF + 0.1 wt. % HNTscompared to 6.9 mg for the unfilled SF. It is also observed that the standard deviation of the wear massresults diminishes with increasing nanoparticle concentration, thus increasing nanolubricant reliability.

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Figure 4. Block mass loss for SF and MO nanofluids with varying concentrations of HNTs.

Measurements of COF over the duration of the run are presented in Figure 5. These were

calculated by taking the friction force measured through a load cell and dividing it by the applied

load of 750 N. For SF nanofluids (Figure 5b) the improvements are significantly larger than the ones

obtained with MO. Figure 5a also shows that HNTs slightly decreased COF over time with

0.05 wt. % and 0.10 wt. % HNTs in MO due to a polishing effect provided by nanoparticles that

reduced the amount of asperities and metal-metal contact. The slight variations (peaks and valleys)

over time found for Figure 5b may be due to the presence of wear debris that affected the

measurement.

Figure 5. Coefficient of friction (COF) behavior as a function of time for: (a) MO + HNT nanofluids;

(b) SF + HNT nanofluids.

Figure 4. Block mass loss for SF and MO nanofluids with varying concentrations of HNTs.

Measurements of COF over the duration of the run are presented in Figure 5. These werecalculated by taking the friction force measured through a load cell and dividing it by the appliedload of 750 N. For SF nanofluids (Figure 5b) the improvements are significantly larger than the onesobtained with MO. Figure 5a also shows that HNTs slightly decreased COF over time with 0.05 wt. %and 0.10 wt. % HNTs in MO due to a polishing effect provided by nanoparticles that reduced theamount of asperities and metal-metal contact. The slight variations (peaks and valleys) over timefound for Figure 5b may be due to the presence of wear debris that affected the measurement.

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Figure 4. Block mass loss for SF and MO nanofluids with varying concentrations of HNTs.

Measurements of COF over the duration of the run are presented in Figure 5. These were

calculated by taking the friction force measured through a load cell and dividing it by the applied

load of 750 N. For SF nanofluids (Figure 5b) the improvements are significantly larger than the ones

obtained with MO. Figure 5a also shows that HNTs slightly decreased COF over time with

0.05 wt. % and 0.10 wt. % HNTs in MO due to a polishing effect provided by nanoparticles that

reduced the amount of asperities and metal-metal contact. The slight variations (peaks and valleys)

over time found for Figure 5b may be due to the presence of wear debris that affected the

measurement.

Figure 5. Coefficient of friction (COF) behavior as a function of time for: (a) MO + HNT nanofluids;

(b) SF + HNT nanofluids. Figure 5. Coefficient of friction (COF) behavior as a function of time for: (a) MO + HNT nanofluids;(b) SF + HNT nanofluids.

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For the average COF measurements in the steady state (Figure 6) SF + HNTs nanofluids exhibitedlower values compared to MO nanofluids, consistent with wear mass loss results. Here, it is evidentthat HNTs showed no major improvements in COF when added to MO. Nanofluids of SF + HNTsshowed a clear tendency to reduce COF with increasing HNTs concentration. A COF of 0.088 wasfound for unfilled SF, whereas SF + 0.1 wt. % HNTs exhibited a COF of 0.06, representing a reductionof 32%.

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For the average COF measurements in the steady state (Figure 6) SF + HNTs nanofluids

exhibited lower values compared to MO nanofluids, consistent with wear mass loss results. Here, it

is evident that HNTs showed no major improvements in COF when added to MO. Nanofluids of SF

+ HNTs showed a clear tendency to reduce COF with increasing HNTs concentration. A COF of

0.088 was found for unfilled SF, whereas SF + 0.1 wt. % HNTs exhibited a COF of 0.06, representing a

reduction of 32%.

Figure 6. Coefficient of friction values for MO and SF nanofluids with varying HNT nanoparticle concentration.

3.2. Surface Analysis

Micrographs of worn surfaces of tested blocks with MO + HNT and SF + HNT suspensions are

shown in Figure 7 and Figure 8, respectively. The worn block in Figure 7a with MO shows pitting

throughout the sample. Fewer regions with pitting are found for Figure 7b with MO + 0.01 wt. %

HNTs. Pitting wear is further reduced with higher nanoparticle content on the worn surfaces

observed in Figure 7c,d with 0.05 wt. % and 0.10 wt. % HNTs.

Figure 8 with SF nanofluids presents overall smoother surfaces compared to MO nanofluids

(Figure 7). Figure 8a (unfilled SF) shows very few regions with pitting and shallow furrows. Lower

groove density with increasing HNT concentration is evident in Figure 8b–d, with a very smooth

surface found with 0.10 wt. % HNTs.

Surface roughness profiles of worn blocks shown in Figure 7 and Figure 8 taken

perpendicularly to the sliding direction of the test are depicted in Figure 9 and Figure 10,

respectively. Table 2 presents surface roughness parameters of Ra, Rq, and Rz characterized by an

optical measurement system. For both MO and SF, the nanofluids roughness parameter values were

very similar, and decreased with increasing HNT concentration. Reductions in Ra by 36% and 34%

were obtained for MO and SF nanofluids, respectively, at 0.10 wt. % HNTs. Lower Ra values

achieved with HNTs were due to a higher amount of removed material during the wear testing that

provided a polishing effect, as observed in Figure 7 and Figure 8. Similar results have been found by

Sia et al. [20] with nanoparticles effectively reducing the Ra of surfaces through metal removal.

Figure 6. Coefficient of friction values for MO and SF nanofluids with varying HNTnanoparticle concentration.

3.2. Surface Analysis

Micrographs of worn surfaces of tested blocks with MO + HNT and SF + HNT suspensions areshown in Figures 7 and 8, respectively. The worn block in Figure 7a with MO shows pitting throughoutthe sample. Fewer regions with pitting are found for Figure 7b with MO + 0.01 wt. % HNTs. Pittingwear is further reduced with higher nanoparticle content on the worn surfaces observed in Figure 7c,dwith 0.05 wt. % and 0.10 wt. % HNTs.

Figure 8 with SF nanofluids presents overall smoother surfaces compared to MO nanofluids(Figure 7). Figure 8a (unfilled SF) shows very few regions with pitting and shallow furrows. Lowergroove density with increasing HNT concentration is evident in Figure 8b–d, with a very smoothsurface found with 0.10 wt. % HNTs.

Surface roughness profiles of worn blocks shown in Figures 7 and 8 taken perpendicularly to thesliding direction of the test are depicted in Figures 9 and 10, respectively. Table 2 presents surfaceroughness parameters of Ra, Rq, and Rz characterized by an optical measurement system. For both MOand SF, the nanofluids roughness parameter values were very similar, and decreased with increasingHNT concentration. Reductions in Ra by 36% and 34% were obtained for MO and SF nanofluids,respectively, at 0.10 wt. % HNTs. Lower Ra values achieved with HNTs were due to a higher amountof removed material during the wear testing that provided a polishing effect, as observed in Figures 7and 8. Similar results have been found by Sia et al. [20] with nanoparticles effectively reducing the Ra

of surfaces through metal removal.

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Figure 7. Scanning electron micrographs of blocks after tribological testing of: (a) MO, (b) MO + 0.01

wt. % HNTs, (c) MO + 0.05 wt. % HNTs, (d) MO + 0.10 wt. % HNTs. Arrows indicate the direction of

sliding.

Figure 7. Scanning electron micrographs of blocks after tribological testing of: (a) MO, (b) MO +0.01 wt. % HNTs, (c) MO + 0.05 wt. % HNTs, (d) MO + 0.10 wt. % HNTs. Arrows indicate the directionof sliding.

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Figure 7. Scanning electron micrographs of blocks after tribological testing of: (a) MO, (b) MO + 0.01

wt. % HNTs, (c) MO + 0.05 wt. % HNTs, (d) MO + 0.10 wt. % HNTs. Arrows indicate the direction of

sliding.

Figure 8. Cont.

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Figure 8. Scanning electron micrographs after tribological testing of: (a) SF; (b) SF + 0.01 wt. % HNTs;

(c) SF + 0.05 wt. % HNTs; (d) SF + 0.10 wt. % HNTs. Arrows indicate the direction of sliding.

Elemental analysis performed by EDS on the worn blocks tested with MO + 0.10 wt. % HNTs

and SF + 0.10 wt. % HNTs can be observed in Figure 11a,b, respectively. No Al and/or Si elements

were detected on the surface, suggesting that at the contact pressure of this test of 0.5 GPa there was

no HNT deposition and formation of a protective film. Furthermore, nanoparticles were too large to

fill valleys or grooves of surfaces and provide a mending tribological mechanism.

This is in contrast to previously reported results by our group [33], where at 3.5–3.9 GPa (in a

four-ball extreme pressure test) Al and Si groups were found on the surface, likely due to HNT

exfoliation due to the compression forces. As explained by Lecouvet et al. [34], HNTs are hard

aluminosilicates with interlayer water molecules where weak Van der Waals forces and H-bonds are

present, and shearing occurs due to the slip motion between these layers, particularly for larger

nanoparticle diameters (>120 nm). A similar behavior has also been reported experimentally by

Lahouij et al. [35] for inorganic fullerene-like (IF)-MoS2 nanoparticles. At contact pressures of 1 GPa

exfoliation of outer fullerene sheets was found to be the predominant mechanism, and at lower

pressures (0.1 GPa) a rolling/sliding mechanism was observed. It is explained that under

compression forces the IF nanoparticles gradually deform and start to exfoliate, which is enhanced

by the large number of defects in the layers.

It is then proposed that the enhancement in tribological properties provided by HNTs can be

attributed to a third body mechanism, where nanoparticles separate surfaces reducing the real area

of contact and reducing wear, as proposed by Ghaednia et al. [36], and their possible sliding between

surfaces. The schematic of the anti-wear mechanism provided by HNTs is shown in Figure 12. As

observed in Figure 4 and Figure 5, increasing HNTs concentration resulted in lower wear mass loss

due to more nanoparticles on the wear zone that were able to reduce the area of contact and

decreased friction and wear. For MO nanofluids the lower stability of HNTs shown in Figure 2 likely

caused more precipitation, thus fewer nanoparticles were able to separate surfaces explaining their

poorer performance compared to SF nanofluids.

Figure 8. Scanning electron micrographs after tribological testing of: (a) SF; (b) SF + 0.01 wt. % HNTs;(c) SF + 0.05 wt. % HNTs; (d) SF + 0.10 wt. % HNTs. Arrows indicate the direction of sliding.

Elemental analysis performed by EDS on the worn blocks tested with MO + 0.10 wt. % HNTsand SF + 0.10 wt. % HNTs can be observed in Figure 11a,b, respectively. No Al and/or Si elementswere detected on the surface, suggesting that at the contact pressure of this test of 0.5 GPa there wasno HNT deposition and formation of a protective film. Furthermore, nanoparticles were too large tofill valleys or grooves of surfaces and provide a mending tribological mechanism.

This is in contrast to previously reported results by our group [33], where at 3.5–3.9 GPa (in afour-ball extreme pressure test) Al and Si groups were found on the surface, likely due to HNTexfoliation due to the compression forces. As explained by Lecouvet et al. [34], HNTs are hardaluminosilicates with interlayer water molecules where weak Van der Waals forces and H-bondsare present, and shearing occurs due to the slip motion between these layers, particularly for largernanoparticle diameters (>120 nm). A similar behavior has also been reported experimentally byLahouij et al. [35] for inorganic fullerene-like (IF)-MoS2 nanoparticles. At contact pressures of 1 GPaexfoliation of outer fullerene sheets was found to be the predominant mechanism, and at lowerpressures (0.1 GPa) a rolling/sliding mechanism was observed. It is explained that under compressionforces the IF nanoparticles gradually deform and start to exfoliate, which is enhanced by the largenumber of defects in the layers.

It is then proposed that the enhancement in tribological properties provided by HNTs can beattributed to a third body mechanism, where nanoparticles separate surfaces reducing the real area ofcontact and reducing wear, as proposed by Ghaednia et al. [36], and their possible sliding betweensurfaces. The schematic of the anti-wear mechanism provided by HNTs is shown in Figure 12.As observed in Figures 4 and 5, increasing HNTs concentration resulted in lower wear mass loss dueto more nanoparticles on the wear zone that were able to reduce the area of contact and decreasedfriction and wear. For MO nanofluids the lower stability of HNTs shown in Figure 2 likely causedmore precipitation, thus fewer nanoparticles were able to separate surfaces explaining their poorerperformance compared to SF nanofluids.

Appl. Sci. 2017, 7, 1019 10 of 15

Appl. Sci. 2017, 7, 1019 10 of 15

Figure 9. Roughness profiles and Ra values of worn blocks for MO + HNT nanofluidos: (a) MO, (b) MO +

0.01 wt. % HNTs, (c) MO + 0.05 wt. % HNTs, (d) MO + 0.10 wt. % HNTs. Figure 9. Roughness profiles and Ra values of worn blocks for MO + HNT nanofluidos: (a) MO,(b) MO + 0.01 wt. % HNTs, (c) MO + 0.05 wt. % HNTs, (d) MO + 0.10 wt. % HNTs.

Appl. Sci. 2017, 7, 1019 11 of 15

Appl. Sci. 2017, 7, 1019 11 of 15

Figure 10. Roughness profiles and Ra values of worn blocks for SF + HNT nanofluidos: (a) SF;(b) SF + 0.01 wt. % HNTs; (c) SF + 0.05 wt. % HNTs; (d) SF + 0.10 wt. % HNTs. Arrows indicate thedirection of sliding.

Appl. Sci. 2017, 7, 1019 12 of 15

Table 2. Comparison of surface roughness characteristics of block materials.

Average Roughness,Ra (µm)

Root Mean Square,Rq (µm)

Mean RoughnessDepth, Rz (µm)

MO 0.53 0.80 4.47MO + 0.01 wt. % 0.44 0.58 2.92MO + 0.05 wt. % 0.37 0.52 3.15MO + 0.10 wt. % 0.34 0.46 2.75

SF 0.53 0.68 3.29SF + 0.01 wt. % 0.47 0.63 2.83SF + 0.05 wt. % 0.42 0.61 2.94SF + 0.10 wt. % 0.35 0.46 2.48

Appl. Sci. 2017, 7, 1019 12 of 15

Figure 10. Roughness profiles and Ra values of worn blocks for SF + HNT nanofluidos: (a) SF; (b) SF

+ 0.01 wt. % HNTs; (c) SF + 0.05 wt. % HNTs; (d) SF + 0.10 wt. % HNTs. Arrows indicate the direction

of sliding.

Table 2. Comparison of surface roughness characteristics of block materials.

Average Roughness,

Ra (µm)

Root Mean Square,

Rq (µm)

Mean Roughness Depth,

Rz (µm)

MO 0.53 0.80 4.47

MO + 0.01 wt. % 0.44 0.58 2.92

MO + 0.05 wt. % 0.37 0.52 3.15

MO + 0.10 wt. % 0.34 0.46 2.75

SF 0.53 0.68 3.29

SF + 0.01 wt. % 0.47 0.63 2.83

SF + 0.05 wt. % 0.42 0.61 2.94

SF + 0.10 wt. % 0.35 0.46 2.48

Figure 11. Elemental analysis of worn blocks after tribological testing for: (a) MO + 0.10 wt. % HNTs,

and; (b) SF + 0.10 HNTs. Figure 11. Elemental analysis of worn blocks after tribological testing for: (a) MO + 0.10 wt. % HNTs,and; (b) SF + 0.10 HNTs.

Appl. Sci. 2017, 7, 1019 13 of 15

Appl. Sci. 2017, 7, 1019 13 of 15

Figure 12. Schematic of anti-wear mechanism for HNTs nanofluids. A reduction of the real area of

contact and sliding between surfaces with no deposition is proposed.

Overall, the significant difference in performance per concentration between the nanofluids of

MO and SF are due to the different anti-wear characteristics of each base fluid, and physical

properties such as fluid viscosity. The results shown in this study suggest that eco-friendly HNTs

have a positive effect improving tribological properties of MWFs. Surface roughness and wear mass

loss may be lowered due to a reduction of the contact area between surfaces by HNTs, thus

extending tool life, reducing material costs, and improving the surface finish of products.

4. Conclusions

In this work, two MWFs filled with low concentrations of HNT additives were prepared and

their anti-wear tribological properties were characterized with a T-05 block-on-ring tribotester.

Nanoparticle additives effectively reduced wear mass loss for both MO and SF nanofluids, with

improvements of up to 64%. Surface roughness of blocks and COF were also lowered. At the contact

pressure of the test of 0.5 GPa HNTs showed no exfoliation, thus the enhancement in anti-wear

properties is attributed to a reduction of contact area.

Acknowledgments: This works was partially supported by Vicerrectoría Académica Universidad de

Monterrey research grant No. 15013. The authors would like to acknowledge Manuel Morales for his

contribution to this work.

Author Contributions: L.P.-P. conceived and designed the experiments and wrote the paper; J.A.S.-F.

contributed materials, performed EDS experiments, and analyzed the data; C.R.M., J.A.O., K.I.S., L.M.U.,

M.J.A., and P.G.-P. performed tribological and surface roughness characterization experiments and analyzed

the data; B.C. performed SEM and surface roughness characterization and analyzed the data.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Brinksmeier, E.; Meyer, D.; Huesmann-Cordes, A.G.; Herrmann, C. Metalworking fluids—Mechanisms

and performance. CIRP Ann. 2015, doi:10.1016/j.cirp.2015.05.003.

2. Schwarz, M.; Dado, M.; Hnilica, R.; Veverkoná, D. Environmental and health aspects of metalworking

fluid use. Polish J. Environ. Stud. 2015, 24, 37–45.

3. Do, T.-V.; Hsu, Q.-C. Optimization of Minimum Quantity Lubricant Conditions and Cutting Parameters in

Hard Milling of AISI H13 Steel. Appl. Sci. 2016, 6, 83, doi:10.3390/app6030083.

4. Gariani, S.; Shyha, I.; Inam, F.; Huo, D. Evaluation of a Novel Controlled Cutting Fluid Impinging Supply

System When Machining Titanium Alloys. Appl. Sci. 2017, 7, 560, doi:10.3390/app7060560.

5. Wu, Y.Y.; Tsui, W.C.; Liu, T.C. Experimental analysis of tribological properties of lubricating oils with

nanoparticle additives. Wear 2007, 262, 819–825, doi:10.1016/j.wear.2006.08.021.

Figure 12. Schematic of anti-wear mechanism for HNTs nanofluids. A reduction of the real area ofcontact and sliding between surfaces with no deposition is proposed.

Overall, the significant difference in performance per concentration between the nanofluids ofMO and SF are due to the different anti-wear characteristics of each base fluid, and physical propertiessuch as fluid viscosity. The results shown in this study suggest that eco-friendly HNTs have a positiveeffect improving tribological properties of MWFs. Surface roughness and wear mass loss may belowered due to a reduction of the contact area between surfaces by HNTs, thus extending tool life,reducing material costs, and improving the surface finish of products.

4. Conclusions

In this work, two MWFs filled with low concentrations of HNT additives were prepared and theiranti-wear tribological properties were characterized with a T-05 block-on-ring tribotester. Nanoparticleadditives effectively reduced wear mass loss for both MO and SF nanofluids, with improvements ofup to 64%. Surface roughness of blocks and COF were also lowered. At the contact pressure of the testof 0.5 GPa HNTs showed no exfoliation, thus the enhancement in anti-wear properties is attributed toa reduction of contact area.

Acknowledgments: This works was partially supported by Vicerrectoría Académica Universidad de Monterreyresearch grant No. 15013. The authors would like to acknowledge Manuel Morales for his contribution tothis work.

Author Contributions: L.P.-P. conceived and designed the experiments and wrote the paper; J.A.S.-F. contributedmaterials, performed EDS experiments, and analyzed the data; C.R.M., J.A.O., K.I.S., L.M.U., M.J.A., and P.G.-P.performed tribological and surface roughness characterization experiments and analyzed the data; B.C. performedSEM and surface roughness characterization and analyzed the data.

Conflicts of Interest: The authors declare no conflict of interest.

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